An X-ray tube, in which X-rays output, comprises an anode target. Electron beams collide with the anode target to produce X-rays.
X-ray apparatuses incorporating the X-ray tube are utilized for many purposes such as medical diagnosis and industrial nondestructive testing or materials analysis.
In a rotating anode X-ray tube, electrons emitted from a fixed cathode are accelerated and focused by a potential gradient between the cathode and a rotating anode target. The electrons colliding with the surface of the anode target typically with kinetic energy of 20 to 150 keV by the acceleration. Thus, a focal point which becomes the source of X-rays is formed on the target surface.
When such high-kinetic energy electron beams strike the anode target, they are rapidly decelerated by the target material, and thus, X-rays are emitted from the focal point. The target surface comprises a metal having a high melting point such as tungsten or a tungsten alloy. The target surface is formed on a substrate (target main body) comprising a metal having a high melting point such as molybdenum or a molybdenum alloy. Particularly, in the case of an X-ray tube for computed tomography or angiography, which requires the use of high-strengthened electron beams, etc., the temperature of or thermal stress on the substrate during use becomes high. Since, a carbide-strengthened molybdenum alloy such as titanium zirconium molybdenum (TZM) is employed for the substrate. The proportion of the kinetic energy of the electrons striking the anode target that is converted into X-rays is very small at approximately 1%. The rest of the kinetic energy is converted into heat.
In order to easily diffuse the heat produced in an anode target, a thermal radiation film is formed on a part of the top surface of the anode target. The thermal radiation film is generally formed of a metallic oxide composite such as titanium oxide and alumina using, for example, the plasma-spray technique.
However, the amount of gaseous CO and CO2 produced during use is large for an anode target having the aforementioned thermal radiation film formed of metallic oxide such as titanium oxide and alumina on the top surface of a higher carbon-element content molybdenum alloy such as TZM. The gasses produced are gradually released into the vacuum within the X-ray tube and ultimately allow an electric discharge to occur in the tube. As a result, the working life of the X-ray tube is shortened.
As suggested in the Jpn. Pat. Appln. KOKAI Publication No. 05-205675, CO gas are presumed to be produced by the chemical reaction between the carbon or metallic carbide in TZM and the metallic oxide composing the thermal radiation film. As a structure to prevent this reaction, the KOKAI Publication No. 05-205675 discloses a structure of forming a reactive barrier layer that forms carbide by the reaction with carbon in the TZM substrate between the TZM substrate and the thermal radiation film by the plasma-spray technique. The KOKAI Publication No. 05-205675 also discloses a structure of forming a protective coating that is thinner than the reactive barrier layer between the reactive barrier layer and the thermal radiation film in order to further enhance the reliability.